The present invention relates to improved transducers for generating and detecting acoustical energy in solid and semi-solid materials and more particularly to a transducer for the improved generation and detection of shear waves.
Generally, in the study of certain materials, the characteristics of those materials and various physical parameters associated therewith can be observed and measured by an analysis of the propagation of acoustical energy therethrough. For this purpose, numerous types of transducers have been proposed to provide the most effective and efficient transfer and measurement of acoustical energy in such materials. In particular, the techniques of the prior art have employed transducers which generate acoustical energy in the form of shear waves and provide certain advantages over other types of acoustical waves in the study of material characteristics.
As is known, shear waves can be propagated in a solid material by inducing a stress in the solid perpendicular to the direction of the intended propagation. The stress is then propagated in the material with a direction of polarization coincident with the direction of the applied stress. In the prior art, such shear waves have been generated by piezoelectric transducer devices which generate movement in a piezoelectric material by the application of an electric potential thereto. When the appropriate electrical connections are made, the stress is transmitted by movement of the piezoelectric element in contact with the material in which it is desired to induce the shear wave. In such instances, a shear wave is generated in a direction perpendicular to the direction of movement of the piezoelectric surface. In still other instances, piezoelectric elements are used to move a member attached thereto. When such member is subsequently placed in contact with a material in which it is desired to induce shear waves, the waves are again generated in a manner similar to that previously described.
In order to effectively study materials using shear waves, it is important that the acoustical impedance of the transducer be closely matched to the acoustical impedance of the material under study to insure sufficient coupling of acoustical energy therebetween. Using the above-noted prior art techniques, successful coupling of the acoustical energy in the form of shear waves has been achieved when using materials exhibiting a high shear modulus. In those instances, the acoustical impedance of the shear wave transducer has been closely matched to the acoustical impedance of the material under study. However, when working with materials exhibiting a low shear modulus, problems have developed due to an impedance mismatch. In particular, the velocity of shear wave propagation in such materials is so low that the mismatch with the transducer precludes coupling of sufficient energy through the material for detection by a receiving transducer. In addition, attenuation of the shear wave is very high in the material and increases with frequency, thereby effectively preventing propagation of a shear wave over any significant distance. Consequently, the analysis of certain materials is severely restricted and any measurements made are subject to inaccuracies caused by the lack of effective energy coupling.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above-noted and similar techniques and to provide a more reliable and versatile transducer for the generation and detection of shear waves.